In general, a percolated layer is a non-continuous metal layer comprising metal nanoparticles interconnected one to the other so as to ensure electric conduction.
In the case of two-dimensional percolated layers, metal nanoparticles are distributed on a single plane and interconnected one to the other so as to ensure electric conduction on the plane of the layer. In another type of percolated layer, known as three-dimensional layer, metal nanoparticles are distributed on a generic three-dimensional structure instead of a single plane.
Three-dimensional percolated structures are generally obtained by supramolecular templating techniques, which generally make use of asymmetric organic molecules as templating elements, to be removed once the metal nanoporous structure has been obtained.
The interface metal-insulator is a typical situation within a metal system under percolation, which can be met for every discontinuity of the system itself.
There are various mechanisms of electron transport through an interface metal-insulator-metal, namely ohmic conduction, ionic conduction, heat emission, emission by field effect. In a given material each of the aforesaid mechanisms dominates within a given temperature and voltage range (electric field) and has a characteristic dependence on current, voltage and temperature. These various processes are not necessarily independent one from the other.
Emission by field effect, also known as Fowler-Nordheim electron tunneling effect, consists in electron transport through an interface metal-insulator-metal due to tunnel effect. Said phenomenon takes place in the presence of strong electric fields, which can bend the energy bands of the insulator means until a narrow triangular potential barrier is built between metal and insulator. The density of emission current by field effect strongly depends on the intensity of the electric field, whereas it is basically independent from temperature, according to the following function:
  j  =            C      ϕ        ⁢          (              β        ⁢                                  ⁢                  E          2                    )        ⁢          exp      ⁡              (                  -                                    B              ⁢                                                          ⁢                              ϕ                                  3                  /                  2                                                                                    β                ⁢                                                                  ⁢                E                            ⁢                                                                                  )            
where E is the intensity of the electric field, φ is the height of the potential barrier, B, C and β are constants.
The probability of tunneling for the electrons of Fermi Level is very low unless the barrier has a thickness below 10 Å. The critical value of the electric field above which emission by tunneling effect takes place is of about 109 volt/meter.
Within a percolated metal system, and namely on every interface metal-void, there are local increases of electric field, such as to reach values of electric field intensity that are necessary for electron tunneling effect. On every discontinuity of the percolated metal system, where there is a local increase of electric field and electron emission by field effect takes place, a local increase of current density can be observed. As a matter of fact, electrons emitted by field effect, as well as those deriving from heat emission, contribute to total electric current. For this reason the percolated metal system shows a voltage-current characteristic with a non-ohmic development: the increase of current with the on applied voltage, thanks to heat emission and to emission by field effect, is faster than in an ohmic conductor with a linear characteristic.
The present Applicant has previously suggested to exploit the electron tunneling effect that can be obtained in a percolated metal structure so as to excite luminescent nanoparticles present within the structure. To this purpose, document WO03058728 describes an electroluminescent device comprising:                a glass or plastic supporting substrate;        at least two electrodes placed on the substrate;        a plurality of luminescent inclusions housed in respective cavities of the three-dimensional percolated layer,        
in which the luminescent inclusions are operative to emit light when excited by electrons getting through the three-dimensional percolated layer by electron tunneling effect.